Gabriel Monod 
Medieval Europe [EPUB ebook] 

No sudden invasion cast the barbarian peoples of Germany on the provinces of the Empire at the end of the fourth century. One has only to recall the long exodus of the Cimbri and the Teutones, the destruction of the Suevi by Cæsar, the struggles of Drusus, of Germanicus, and of Tiberius against the Chatti, Cherusci, and the Marcomanni. At first the Romans had the advantage. The legions crossed the natural limits even of the Empire, and to control better the peoples whom they could not conquer they built an extended line of entrenchments…